It is known from the prior art (e.g., EP 2 203 033 A2) to vaporize liquid or solid emitter materials by means of a beam of high-energy radiation for generating a gas discharge plasma emitting EUV radiation. This vaporization is carried out in a discharge space between two electrodes to which a pulsed high voltage is applied in order to generate a discharge current through the vaporized emitter material in such a way that the emitter material is converted as completely as possible into a gas discharge plasma.
The emitter material can be fixedly arranged on the surface of the electrodes or, as is described in DE 10 2005 039 849 A1, can be continuously applied as a melt to electrodes which are constructed as rotating electrodes, a portion of whose circumference is immersed, respectively, in a bath with molten emitter materials.
Further, it is known to inject emitter materials in a regular sequence of droplets between the electrodes as is also described, e.g., in DE 10 2005 039 849 A1. The distance between the electrodes and the location of plasma generation can be maximized by means of a solution of this kind so that the lifetime of the electrodes is increased.
When the emitter material is injected in droplets, the buffer gas, which usually serves to brake the high-energy particles developing in plasma generation (debris mitigation), moreover in ionized form, acts as an electrically conducting medium. This conducting medium is used to supply a droplet of emitter material with the electric power necessary for heating and for the generation of a plasma.
This has the disadvantage that the ionized buffer gas and possibly also gaseous residues of emitter material originating from previous discharges are widely distributed in the discharge space, as a result of which the discharge current between the electrodes does not flow in a targeted manner through a selected droplet of emitter material but, rather, a substantial proportion of the discharge current flows around the emitter material droplet. Because of this effect, the conversion efficiency, i.e., the ratio of energy used to the EVU radiation energy generated, remains low.
EP 2 051 140 A1 discloses a method and a device by which an electrically conductive discharge region is generated between two disk-shaped electrodes in a discharge space. To this end, a pulsed high-energy beam is directed into a focus with a defined focus length. This focus length extends perpendicular to the desired path of the discharge current and a high excitation energy is supplied along the entire focus length between the electrodes. An emitter material is supplied at a certain distance (Rayleigh range) from the desired discharge channel and is vaporized by the action of the excitation beam. The mixture of vaporized emitter materials and buffer gas formed in this way arrives in the discharge space between the electrodes. By applying a pulse of the excitation beam to the gas again in a suitably timed manner, the ionized residual gas is further excited in the area in which the discharge channel is to be generated and, at the same time, a voltage pulse is supplied to the electrodes causing an electrically conductive discharge channel for the electric discharge between the electrodes and the formation of a gas discharge plasma.
This has the disadvantage that an excitation of the ionized residual gas over the entire electrode spacing is impossible because of the beam geometry that must be maintained. Further, because of the large focus length of the excitation beam within the discharge space, there is a high buffer gas ionization between the electrode surfaces over relatively large areas, which impedes the formation of a narrowly circumscribed discharge channel.